Solvent-polarity reconfigurable fluorescent 4-piperazino-N-aryl-1,8-naphthalimide crown ether logic gates

Abstract

Four compounds 1–4 were designed and synthesised, comprising a 4-amino-N-aryl-1,8-naphthalimide fluorophore, a piperazine receptor, and an aryl group, as fluorescent logic gates. At the imide position, the substituent is phenyl (1), 1,2-dimethoxyphenyl (2), benzo-15-crown-5 (3), or benzo-18-crown-6 (4). Molecules 1 and 2 are constructed according to a fluorophore–spacer–receptor format, while 3 and 4 are engineered according to a receptor₁–spacer₁–fluorophore–spacer₂–receptor₂ format based on photoinduced electron transfer and internal charge transfer mechanisms. The compounds were studied in water, water/methanol mixtures of different ratios, and methanol by UV-visible absorption and steady-state fluorescence spectroscopy, as a function of pH, metal ions and solvent polarity. The excited state of 1–4 is 8.4 ± 0.2 in water, 7.6 ± 0.1 in 1 : 1 (v/v) water/methanol, and 7.1 ± 0.3 in methanol. The of 3 in water is 0.92 and the and of 4 in water are 2.3 and 2.9. ¹H NMR data in D₂O and CD₃OD confirm H⁺ interaction at the piperazine moiety, and Na⁺ and Ba²⁺ binding at the benzo-15-crown-5 and benzo-18-crown-6 moieties of 3 and 4. By altering the solvent polarity, the fluorescent logic gates can be reconfigured between TRANSFER logic and AND logic. Molecules with polarity reconfigurable logic could be useful tools for probing the microenvironment of cellular membranes and protein interfaces.